In order to grow plants in greenhouses, a proper combination of light, heat, and humidity must be maintained. This combination of light, heat, and humidity should be consistent throughout the greenhouse so that plants grow uniformly regardless of position therein. At times, the requisite light and heat are provided naturally from the sun, which projects radiant energy through the elevated window areas of the greenhouse. However, it is often too cold during the winter to grow plants without some source of additional heat, especially in northern climates.
Heat may be provided by forced air heaters which distribute heated air directly into the greenhouse. However, this arrangement is not entirely satisfactory for many reasons. A forced air system unevenly distributes heated air initially in the greenhouse, and tends to concentrate the heat in the areas around the heated air outlets. Further, much of the hot air is lost through the roof and walls of the greenhouse before being dispersed throughout the greenhouse. As a result, during the winter days, the temperature in many areas of the greenhouse can be as low as 50 degrees which results in improper plant development.
Insulating materials have been used on the roof and walls of the greenhouse to address the previously mentioned heat loss problems associated with direct air heating. A system of this type is disclosed in Cary U.S. Pat. No. 4,064,648, and Ward et al U.S. Pat. No. 4,313,650. Such systems typically provide for reflective insulating panels which, at times, may block direct sunlight from entering the greenhouse and produce unwanted shadows therein. Such systems are also unsatisfactory since the plants need uniform heat and light for proper growth.
Radiant heating systems have been used to heat greenhouses. Radiant systems provide an indirect source of heat and are generally capable of having heating elements which can be more precisely positioned within a greenhouse. Radiant heating systems typically employ metallic energy-emitting tubular conduits mounted overhead in an area to be heated. Such systems further include a burner which fires a heated effluent into the conduit, an opaque reflector mounted over the conduit, and an exhaust system to expel the effluent and products of combustion outside the greenhouse. One such radiant heater system is disclosed in Johnson U.S. Pat. No. 3,399,833.
A characteristic of these radiant heating systems is the wide variance in the amount of thermal energy emitted from the conduit over its working length. While the temperature inside the metallic conduit immediately adjacent the burner may reach levels in excess of 1600.degree. F., the temperature inside the conduit at the exhaust end, depending on length, may be as low as 200.degree. F. This results in an uneven emission of thermal energy over the length of the conduit. Unless the spacing of the tube from the area being heated is varied, there are correspondingly uneven temperatures in the area being heated by the system. This is especially true with low overhead mounted (7'-9') systems in high heat loss structures, such as greenhouses.
One attempt to more evenly distribute the thermal energy radiated by the metallic conduit is disclosed in Prince U.S. Pat. No. 4,319,125. Prince employs a dispersing reflector adjacent to the relatively hot initial portion of the metallic conduit and a parabolic or concentrating reflector adjacent to the colder end portion of the conduit to compensate for the varying intensity of thermal energy radiated over the length of the conduit. While the shaped reflector may provide some measure of improvement in the distribution of the thermal energy throughout the area serviced by the heater system, the opaque conduit and reflector obstruct sunlight entering the greenhouse and produce a shadow which adversely affects the growth of plants thereunder.